1. Field of the Invention
The present invention relates to nuclear radiation measurements by the use of scintillation detectors and in particular to a radiometric method for determining concentration of naturally occurring isotopes of radium and a device therefor.
The present invention may be used in radiogeochemical prospecting and exploration of mineral resources. The invention may also be used to effect radiation monitoring and ecological investigation of an environment in exploiting radioactive deposits, at processing plants and at nuclear power stations, primarily for measuring a natural radionuclide background in industrial areas and discharges of radioactive elements into the atmosphere, as well as water pollution. The invention is suitable for measuring concentration of radionuclides in construction materials in utilizing mining-industry waste. It may be advantageously used in solving various meteorological problems, say, for determining the age and origin of an air mass in Rn and ThB tests, as well as in health resorts (for radon and emanation treatment), another possible application of the invention being measurements of radionuclide concentration in dry products.
2. Prior Art
A widely known prior art method for determining concentration of naturally occurring isotopes of radium in samples (cf. M. Curie "Radioactivity", State Phys. Math. publishing house, Moscow, 1960, pp. 157-162) comprises the steps of full chemical decomposition of a sample of an analyzed substance, separation of isotopes of radium from the solution, re-solution of radium, sealing of the solution in a bubbler to build up emanation over a predetermined time interval equal to an emanation build-up time, transfer of emanation into a measuring chamber, and a subsequent measurement of gas .alpha.-radiation activity. Losses due to radium absorption on walls of glass flasks lead to appreciable systematic errors in measurements, a disadvantage substantially limiting the use of the aforesaid method. Moreover, the foregoing method has been generally unsatisfactory due to a comparatively long analysis time. (It takes at least 7-9 days to obtain final results after the analysis is started). The sample of material is chemically decomposed whereby the analysis may not be repeated.
Also known in the art is a radiometric method of determining .gamma.-radiation concentration of isotopes of radium (cf. E. I. Zheleznov, I. P. Shumilin and B. Ya. Yufa "Radiometric Methods of Analyzing Natural Radioactive Elements", Nedra, Moscow, 1968, pp. 197-200, in Russian), which comprises the steps of placing a sample into a hermetically sealed container, emanation build-up at a preset time interval, and a subsequent .gamma.-spectrometric measurement by the use of a NaI (Tl) radon decay product .gamma.-radiation scintillation crystal.
The aforesaid method does not provide a desired measuring accuracy in the case of a high thorium concentration, particularly with an upset radioactive equilibrium in a .sup.232 Th-MsTh series, and also when a potassium concentration is high (.sup.40 K due to .gamma.-radiation). To account for non-radium radiator .gamma.-radiation contribution, it is necessary to additionally analyze the sample by the X-ray spectral method (U and Th) and by the flame photometric method (K). Such a method does not permit determining isotopes of radium for low-mass samples (less than 30 g).
Also known in the art is a radiometric method of determining naturally occurring isotopes of radium (cf. A. L. Yakubovich, M. Ye. Kotsen "Selective Analysis of Radionuclides by the Delayed Coincidence Method", Journal of Radioanalytical Chemistry, Vol. 57, No. 2, 1980, pp. 461-472) which comprises the steps of placing a sample into an open dish, and positioning the dish with the sample allowing a small air gap under a combination scintillator (polystyrene with p-terphenyl and 1.4-bis-5 phenyloxazolyl-benzol, and ZnS (Ag) simultaneously recording .beta.- and .alpha.-radiation of the samples and .beta.-.alpha. delayed coincidences of radon and thoron decay products (radionuclides of the radium and ThX family). With the aforesaid method, samples should be measured in layers of a different thickness (in three dishes varying as to depth) to determine an emanation loss factor which must be accounted for in measuring radium concentration. However, this factor may be determined to a low accuracy due to the fact that emanation separation is affected by atmospheric pressure variations, air temperature fluctuations, and a moisture content of samples. The foregoing method does not allow measurements of low-mass samples (less than 15 g) due to the fact that each sample has to be placed in three dishes varying as to depth. The measurement results are appreciably affected by the radioactive equilibrium in the Th-MsTh-ThB series and the .sup.238 U-.sup.234 U-10 series since these radionuclides are .alpha.-radiators and the emanation loss factor is computed by solving a balance equation in equilibrium systems of natural radioactive families with .alpha.-radiation.
A device for executing the aforesaid method (A. L. Yakubovich, M. Ye. Kotsen "Selective Analysis of Radionuclides by the Delayed Coincidence Method", Journal of Radioanalytical Chemistry, Vol. 57, No. 2, 1980, pp. 461-472) represents a beta-alpha radiometer. The device comprises a combination scintillator disposed in close proximity to a sample of an analyzed substance, and a sample feed mechanism, into which three dishes containing the sample are successively installed, said dishes being open at the top. A photomultiplier tube recording scintillation is connected to an electronic selector separating pulses corresponding to .beta.- and .alpha.-particles and .beta.-.alpha. cascade pairs of delayed coincidences of RaC and ThC radionuclides, said selector being connected to a recording element.
The known device does not allow measurements of hermetically sealed samples due to low sensitivity associated with 2.pi. measuring geometry. Such a device does not permit radium concentration measurements in a sample wherein radium concentration is less than 10.sup.-10 % of a radium mass fraction.